Add the fetchpriority="" attribute

This draws from https://wicg.github.io/priority-hints/#html-integration, adding the fetchpriority="" attribute (and fetchPriority property) to <img>, <link>, and <script>.

<iframe> is intentionally omitted as it's not clear the semantics apply that well; see #8470 (comment).

Closes #7150.

Author: pmeenan